Manufacture of strong nitric acid

ABSTRACT

Nitric acid having a strength of about 90 to 100% HNO.sub.3, by weight, is prepared by countercurrently absorbing gaseous nitrogen peroxide in cold, strong nitric acid whose concentration is about from 75 to 85% HNO.sub.3, by weight, thereby forming a true solution of the strong acid and nitrogen peroxide; reacting the true solution with molecular oxygen and water at high pressure, thereby increasing the concentration of the strong acid, while recovering nitrogen peroxide desorbed from the solution during the reaction; vaporizing a portion of the solution to recover unreacted nitrogen peroxide; recycling this nitrogen peroxide and that recovered from the reaction step to the absorption step; and distilling the strong acid at about atmospheric pressure to give as distillate the strong nitric acid product, thereby leaving as bottoms the strong acid used to absorb nitrogen peroxide in the first step.

United States Patent n91 Wendel MANUFACTURE OF STRONG NITRIC ACID Martin Maurice Wendel, Glen Mills, Pa.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: Aug. 28, 1972 [21] Appl. No.: 284,l38

Related US. Application Data [63] Continuation-impart of Ser. No. 63,562, Aug. I3, 1970, abandoned. which is a continuation-in-part of Ser. No. 733,406, May 3], 1968, abandoned.

[75] Inventor:

[52] US. Cl 423/392; 423/394 [5|] Int. Cl C0lb 21/40 [58] Field of Search 423/392, 393, 394

[56] References Cited UNITED STATES PATENTS 1,943,963 2/l934 Kramer 423/392 2.088.057 7/l937 Handforth 423/392 FOREIGN PATENTS OR APPLlCATIONS 456,5l8 ll/l956 United Kingdom 423/392 9lO,l3l ll/l962 United Kingdom 423/394 342.068 l/l93l United Kingdom 423/394 OTHER PUBLICATIONS Miles; Nitric Acid Manufacture and Use, Oxford Univ.

[ Apr. 8, 1975 Press, London, I961, pp. 12-13, 33-34, 49-51, and 54.

Primary Examiner-G. O. Peters [57] ABSTRACT Nitric acid having a strength of about 90 to 100% HNO by weight, is prepared by countercurrently absorbing gaseous nitrogen peroxide in cold, strong nitric acid whose concentration is about from 75 to 85% HNO by weight, thereby forming a true solution of the strong acid and nitrogen peroxide; reacting the true solution with molecular oxygen and water at high pressure, thereby increasing the concentration of the strong acid, while recovering nitrogen peroxide desorbed from the solution during the reaction; vaporizing a portion of the solution to recover unreacted nitrogen peroxide; recycling this nitrogen peroxide and that recovered from the reaction step to the absorption step; and distilling the strong acid at about atmospheric pressure to give as distillate the strong nitric acid product, thereby leaving as bottoms the strong acid used to absorb nitrogen peroxide in the first step.

l5 Claims, 2 Drawing Figures MANUFACTURE OF STRONG NITRIC ACID BACKGROUND OF THE INVENTION This invention relates to a process for directly preparing strong nitric acid having a concentration of about from 90 to 100% HNO by weight. This application is a continuation-in-part of my copending application Ser. No. 63,562, filed Aug. 13, 1970, which in turn is a continuation-in-part of my copending application Ser. No. 733,406 filed in May 31, 1968 both now abandoned.

As used herein, nitrogen peroxide" denotes the equilibrium of N0 and N 0 All concentrations hereinafter referred to are weight percent unless otherwise specifically indicated, and the nitric acid concentrations are on a nitrogen peroxide-free basis.

Although most nitric acid currently produced has a concentration less than about 68% HNO there is a substantial need for stronger acid, particularly for acid whose concentration is in the about 90100% HNO range. Such acid is used, for example. in nitration reactions such as in the preparation of explosives, dyestuffs and biochemicals. However, since nitric acid and water form a constant boiling mixture (azeotrope) between about 68 and 69% HNO more concentrated acid cannot be directly prepared by distillation. Accordingly, commercial processes had to be developed, which would circumvent the problem created by the azeotrope. Several processes are being practiced in the industry. For example, strong nitric acid is being made by extractive distillation with sulfuric acid; by modifying the azeotropic composition with magnesium nitrate and then distilling the resultant mixture; and by separating liquid nitrogen tetraoxide from a mixture of nitrogen oxides for subsequent reaction with weak HNO e.g., as in the Bamag and Fauser processes. None of these methods, however, is completely satisfactory from an economic standpoint, and those which produce strong nitric acid most economically are inflexible in the sense that they cannot simultaneously prepare nitric acid of less than about 68% concentration when desired.

US. Pat. No. 2,088,057 Handforth (1937), assigned to the assignee of the present invention, concerns a process for making nitric acid wherein first the water is removed from ammonia oxidation reaction products without removing nitric acid values, and then the remaining gases are reacted with water and oxygen and absorbed at substantially the same pressures, such as 50 atmospheres, to produce nitric acid. The removal of the water involves expenses which would be desirable to be able to avoid.

British Pat. No. 456,518 Davies 1936) uses an ammonia oxidation converter at low pressure, such as atmospheric pressure, and pumps up the nitrogen-oxide containing gases to elevated pressures for reaction with air or oxygen in a multistage column to produce nitric acid. The operation of the ammonia converter at atmospheric pressure facilitates removal of water to permit the nitric acid production, but pumping up the pressure of such corrosive gases is difficult and costly compared to increasing the pressure of liquids. Davies also uses acid with a concentration of over 90% to produce more acid of that same strength. Ordinary stainless steel materials of construction are subject to corrosion in such acids at the recommended temperatures, and modem practice requires more expensive or less versatile materials for these applications.

Thus, there is a need for an improved process that will permit preparation of concentrated nitric acid at a lower mill cost, avoiding disadvantages of the prior art and providing flexibility heretofore lacking in commercial operations.

SUMMARY OF THE INVENTION This invention provides an improved direct process for the production of nitric acid having a concentration of about 90 to 100% HNO and optionally for the simultaneous production of aqueous nitric acid having a concentration of about from 55 to 68% I-INO The improvement resides in a combination of process steps which makes judicious use of process water and nitrogen peroxide, thus effecting substantial savings in the mill cost of the concentrated acid, while permitting, but not requiring in all cases, the simultaneous direct production of nitric acid of less than azeotropic composition. In particular, the process of this invention comprises continuously:

a. countercurrently absorbing gaseous nitrogen peroxide in aqueous nitric acid bottoms from step (e) at a temperature and pressure of about from 0 to 25C. and to 120 psig, respectively, thereby forming a true solution of the aqueous nitric acid having a concentration of about from 75 to HNO on a nitrogen peroxide free basis and about from 15 to 30% dissolved nitrogen peroxide, based on the total weight of the solution;

b. reacting the true solution of step (a) with molecular oxygen and water at a temperature in the range of about 40 to 100C. and a pressure in the range of about 300 to 1.600 psig, thereby increasing the concentration of the aqueous nitric acid of step (a) to about 80 to HNO while recovering gaseous nitrogen peroxide desorbed from the true solution during the reaction and recycling the gaseous peroxide to be absorbed in step (a);

c. vaporizing a portion of the aqueous nitric acid stream from step (b) to remove at least 50% of the ni trogen peroxide contained therein;

d. absorbing the nitrogen peroxide obtained in step (c) in a portion of the bottoms from step (e); and

e. distilling the aqueous nitric acid from step (c) at about atmospheric pressure and recovering as distillate strong nitric acid having a concentration of about from 90 to l-lNO leaving as bottoms aqueous nitric acid having a concentration of about from 75 to 85% HNO respectively; and optionally, individually or collectively,

g. scrubbing the gaseous effluent from absorption step (a) with water to recover unabsorbed gaseous nitrogen peroxide and HNO vapor as weak nitric acid;

h. distilling the weak acid from step (g), or the aqueous nitric condensate having a concentration of less than about 55% NHO or both together, to give as distillate aqueous nitric acid having a concentration below about 5% HNO;, and as bottoms aqueous nitric acid having a concentration of about from 55 to 68.5%

i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as the source of said water;

j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water.

The term "vaporization" as used herein refers to a unit operation wherein dissolved nitrogen peroxide is removed from a liquid as, for example, by bleaching where the peroxide is stripped from the liquid or by flashing.

The foregoing steps of absorption (a), highpressure reaction (b), vaporization (c), and distillation (e) are the nucleus of the process of this invention and give it the desired flexibility. Recovering nitrogen peroxide as in steps (b) and (d) and recycling this nitrogen peroxide to absorption step (a) and recycling the original absorbent from the distillation step (bottoms) to the absorption step are procedures designed to improve the economics of the process. Additional economics may be realized by employing preferred process steps hereinafter described.

By its unique combination of steps, especially in the more preferred embodiments, the present invention offers substantially improved economics by enabling better materials balances and more efficient energy utilization. All of the water used can be from the catalytic oxidation of ammonia which avoids stress-corrosioncausing chlorine found in many water supplies. Also, nitric acid produced according to the invention does not contain sulfates or magnesium salts found in strong nitric acid produced traditionally by extractive distillation using sulfuric acid or magnesium nitrate, respectively.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 and 2 of the accompanying drawings illustrate flow diagrams for two complete preferred embodiments of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION To facilitate understanding of the interrelationship among the various steps of the present process, it will first be described with reference to the complete pre ferred embodiments shown on the drawings, and thereafter, other embodiments will be discussed.

With reference to FIG. 1 ammonia from line 1 and air from line 2 are mixed and fed to a Converter where the ammonia is catalytically oxidized with oxygen from the air. This ammonia oxidation step is conducted at elevated temperatures and pressures in a conventional manner, and preferably at a pressure slightly higher than is to be employed in the Absorber to allow for pressure drop between it and the Converter. Hot gaseous ammonia oxidation products from the Converter are fed (line 3) to the Cooler-Condenser wherein water of reaction is condensed and most nitric oxide (NO) is oxidized to nitrogen peroxide, while the temperature of the gas is maintained above the dew point of the nitrogen peroxide. The resulting two-phase mixture of gas and condensate, which will generally have a temperature of the order of about from lto 70C., enters the Separator wherein condensate is separated from the gas phase as weak aqueous nitric acid and is fed (line 5) to the Weak HNO; Distillation Column. The Cooler- Condenser and Separator preferably are designed to permit removal of water of reaction with minimum absorption of nitrogen peroxide, but line 5 may contain acid having a strength as high as about 55% I-INO Conventional cooler-condenser and separator designs normally give a condensate stream whose strength is about from 35 to 45% HNO The gaseous mixture leaving the Separator (line 4) is substantially anhydrous and contains about from 5 to 10% nitrogen oxides, by volume, balance mainly nitrogen and oxygen. This gaseous mixture is fed to the Absorber together with gaseous nitrogen peroxide recovered from the Reactor (line 7) and Bleacher (line 6). The two nitrogen peroxide-containing streams (4 and 8) mix in the Absorber to give a combined reaction gas containing about from 8 to 13%, and preferably about from 9 to 12%, nitrogen peroxide, by volume.

In this embodiment the Absorber is divided into two sections. In the lower section, which operates at about from 0 to l5C. and about from 80 to I20 psig, a recycle absorbent stream (line 18) from the bottom of the Strong HNO: Distillation Column and containing about from to 85% HNO flows countercurrently to the as cending gas stream. By means of this counter-current flow, gaseous nitroge peroxide is dissolved in the strong acid, thereby forming a true solution, and the nitrogen peroxide content of the combined reaction gas is reduced to below about 0.3%, and preferably below about 0.1%, by volume. Since the 75 to 85% HNO used in the lower section of the Absorber has an HNO vapor pressure of about 0.8 i 02 psig, the gaseous effluent from this section is saturated with HNO This HNO and much of the remaining (undissolved) nitrogen peroxide are recovered by scrubbing the gaseous effluent with weak aqueous nitric acid (less than about 5% HNO;;) which is fed into the upper Absorber section from the Weak HNO Distillation Column. At least about 45% of the nitrogen peroxide gas that enters the upper section of the Absorber is absorbed. Gases that leave the Absorber (line 10) are sent to conventional catalytic abatement units (where required), to power recovery units, and the like for ultimate stack disposal to the atmosphere. The acid from the upper section is added to the Cooler-Condenser condensate (line 5) to be fed through line 11 to the Weak I-INO Distillation Column. The about 75 to 85% HNO stream that leaves the bottom of the Absorber contains about from 15 to 30% dissolved nitrogen peroxide. No liquid nitrogen peroxide phase is present, rather a true solution of nitrogen peroxide in HNO exists.

The true solution prepared in the Absorber is pumped (line 15) through a heater to the Reactor which operates at about from 300 to 1,600 psig, prefer ably about at least 400 and not over 600 psig, and 40C. to l00C. In the Reactor, about 40 to prefereably 60 t 5% of the nitrogen peroxide in solution is converted to l-INO by air oxidation according to the following reaction:

NO 0 /5 H O HNO Aqueous nitric acid containing about from 55 to 68% is fed from the bottom of the Weak HNO Distillation Column (line 14) to the top of the Reactor to supply the water required for the above indicated reaction. The N0 required is, of course, provided by the dissolved nitrogen peroxide. The O, required for the reaction is supplied at Reactor pressure by air (line 20) from a reciprocal compressor, or the like. In the Reactor, the air (0 not only reacts with the dissolved nitrogen peroxide, but also strips unreaeted nitrogen peroxide from the true solution. The aqueous HNO, stream that is fed (line 14) into the top of the Reactor reabsorbs some of the desorbed nitrogen peroxide and carries it back down into the lower part of the Reactor. The liquid effluent from the Reactor (line 16) contains about 85 i 5% HNO; on a nitrogen peroxide-free basis and less than about and preferably less than about 5% dissolved nitrogen peroxide. This dissolved nitrogen peroxide is removed by air (line 19) in a conventional stripping or bleaching operation in the Bleacher after the pressurized Reactor product stream is depressurized to about 130 i 25 psig. Following this procedure substantially all remaining dissolved nitrogen peroxide is recovered. Off-gases from both the Bleacher and Reactor (lines 6 and 7) are combined (line 8) for recycle to the Absorber.

The strong aqueous nitric acid from the Bleacher, which contains about from 80 to 90% HNO flows (line 17) to the Strong HNO Distillation Column wherein it is distilled at approximately atmospheric pressure as a matter of operating convenience. The overhead from the Strong HNO; Distillation Column is the desired product containing about from 90 to 100% l-lNO depending on the strength of acid distilled. This product is condensed and withdrawn through line 23. The bottoms which contain about from 75 to 85% HNO; are cooled and recycled (line 18) to the Absorber.

1n the illustrated process, the fraction of total HNO production that will be obtained as a sidestream product (line 13) containing about from 55 to 68% HNO is determined primarily by the strength of the Cooler- Condenser condensate withdrawn from the Separator in line 5, higher strength condensate requiring sidestream product withdrawal of more of the weaker acid. For any given condensate strength, the ratio of concentrated HNO production to the weaker sidestream production can be varied over a range of about 6.5/1 to 2/1 by using some of the condensate from the Condenser Cooler to supply part of the water needed for the Reactor.

Referring again to the Weak HNO Distillation Column for more detailed consideration of its operation, condensate from the Separator and weak acid from the upper section of the Absorber are rectified into hottoms and distillate at a temperature of about from 1 10 to 121C. (reboiler temperature) and about atmospheric pressure. The bottoms have a concentration of about from 55 to 68% HNO preferably at least about 65% HNO and the distillate has a strength less than about 5%, preferably less than 0.1% HNO As previously indicated, at least part of the bottoms is fed to the Reactor through line 14 to supply the water required for reaction with oxygen and nitrogen peroxide. The distillate is split and used according to process demands for water. Part of the distillate is fed through line 12 to the Absorber as described above and the remainder is discharged from the system through line 22.

With reference to FIG. 2 an especially preferred embodiment of the present invention may be seen. The streams and equipment which are similar to those of FIG. 1 are numbered similarly. The stream (line 3) entering the Separator wherein condensate is separated from the gas phase as weak aqueous nitric acid is the same composition as in FIG. 1. The condensate is fed (line 5) to a Condensate Flasher operated at a temperature of about 100C. and atmospheric pressure. The liquid from the Condensate Flasher is fed (line 27) to a Weak HNO Distillation Column and the vapor is fed (line 26) to the Atmospheric Pressure Absorber. The Cooler-Condenser and Separator preferably are designed to permit removal of water of reaction with minimum absorption of nitrogen peroxide, but line 5 may contain acid having a strength as high as about 55% P11 10 Conventional cooler-condenser and separator designs normally give a condensate stream whose strength is about from 35 to 45% HNO The gaseous mixture leaving the Separator (line 4) is substantially anhydrous and contains about from 5 to 10% nitrogen oxides, by volume. This gaseous mixture is fed to the Pressure Absorber together with the gaseous nitrogen peroxide recovered from the Reactor (line 7). These two nitrogen peroxide-containing streams (lines 4 and 7) mix in the Pressure Absorber to give a combined reaction gas containing about from 8 to 13%, and preferably about from 9 to 12%, nitrogen peroxide, by volume.

The Pressure Absorber, which operates at about from 0 to 25C. and about from to 120 psig, receives a recycle absorbent stream (line 18) from the bottom of the Strong l-lNO Distillation Column containing from about 75 to HNO which flows countercurrently to the ascending gas stream. By means of this countercurrent flow, gaseous nitrogen peroxide is dissolved in the strong acid, thereby forming a true solution, and the nitrogen peroxide content of the combined reactin gas is reduced to below about 0.3% and preferably below about 0.1%, by volume. Gases that leave the Pressure Absorber (line 10) are sent to conventional catalytic abatement units (where required), or to power recovery units and the like for ultimate stack disposal absorption. Optionally, these gase can be treated by to the atmosphere. Optionally, these gases can be treated by absorption in water or the weak HNO overheads from the weak HNO distillation column, such as in the manner shown in FIG. 1 but not in FlG. 2. The stream that leaves the bottom of the Pressure Absorber contains about from 15 to 30% dissolved nitrogen peroxide and 75 to 85% HNO No liquid nitrogen peroxide phase is present, rather a true solution of nitrogen peroxide in HNO exists.

The true solution prepared in the Pressure Absorber is combined with the bottoms from the Atmospheric Absorber (line 28), and the aqueous nitric acid containing 55 to 68% HNO from the bottom of the Weak l-lNO; Distillation Column (line 14) and is pumped (line 29) through a heater to the Reactor which operates as stated hereinabove. The NO, required is, of course, provided by the dissolved nitrogen peroxide. The 0 required for the reaction is supplied at Reactor pressure by air (line 20) from a reciprocal compressor, or the like. 1n the Reactor, the air (0,) not only reacts with the dissolved nitrogen peroxide, but also strips unreacted nitrogen peroxide from the true solution. Some of the desorbed nitrogen peroxide is carried back down into the lower part of the Reactor by the feed stream (line 29). The liquid effluent from the Reactor (line 16) contains about 85 i 5% HNO on a nitrogen peroxide-free basis and less than about 10% and preferably less than about 5% dissolved nitrogen peroxide. At least 50% of this dissolved nitrogen peroxide is removed by flashing the effluent from a pressure of 530 psig to atmospheric pressure into a holdup tank. Preferably, the elfluent is subsequently heated (line 17) to about 74C. and again flashed. The second flashing substantially removes the balance of the nitrogen peroxide and should be used when it is desired that the nitric acid product be clear and not fuming acid. The vapors from the Flasher are fed (line 32) to the Atmospheric Absorber. Substantially all of the dissolved nitrogen peroxide can be recovered in this manner.

The liquid from the Flasher is fed (line 33) to the Strong HNO Distillation Column wherein it is distilled at approximately atmospheric pressure as a matter of operating convenience. The overheads from the Strong I-INO Distillation Column which are partially condensed and withdrawn through line 34 contain about from 90 to 100% l-INO; depending on the strength of acid distilled. The bottoms which contain about from 75 to 85% HNO are cooled. split into two streams (lines 18 and 35) one of which is recycled (line 18) to the Pressure Absorber and the other recycled to the Atmospheric Absorber in a ratio of approximately /1 respectively.

As in the process of FIG. 1 the fraction of total HNO production that will be obtained as a sidestream product (line 13) containing about from 55 to 68% HNO is determined primarily by the strength of the Cooler- Condenser condensate withdrawn from the Separator in line 5. higher strength condensate requiring sidestream product withdrawal of more of the weaker acid. For any given condensate strength. the ratio of concentrated HNO production to the weaker sidestream production can be varied over a range of about 6.5/1 to 2/1 by using some of the condensate from the Cooler Condenser to supply part of the water needed for the Reactor. Approximately 1.52% of the condensate is vaporized in the Condensate Flasher and the vapors directed to the Atmospheric Absorber (line 26).

The Weak HNO Distillation Column operates substantially as in FIG. 1 except that only the liquid from the Condensate Flasher (line 27) is rectified into bottoms and distillate at a temperature of about from 1 to 125C. (reboiler temperature) and about atmospheric pressure. The bottoms have a concentration of about from 55 to 68% HNO preferably at least about 65% HNO and the distillate has a strength less than about 5%. preferably less than 0.1 HNO As previously indicated. at least part of the bottoms is fed to the Reactor through line 14 to supply the water required for reaction with oxygen and nitrogen peroxide. The distillate is used according to process demands for water. and the remainder is discharged from the system through line 22.

The Atmospheric Absorber receives vapors from var ious holdup tanks in the system, e.g., line 38, the partial condensers of the Strong and Weak I-INO Distillation columns. (lines 40 and 39). the Condensate Flasher- Separator (line 26) and the Strong HNO Flasher (line 32). The liquid feed (line 35) is obtained from the bottoms of the Strong HNO Distillation Column and the offgas is discarded after optional catalytic removal of the nitrogen oxides (line 36).

As previously indicated. the steps of absorption, reaction. vaporization and distillation of strong acid are fundamental to the process of this invention. For this reason their more important parameters are summarized and tabulated below for each process illustrated in FIGS. 1 and II. In the following tables, none of the values given are absolutes and therefore they should be prefaced with the word about.

PROCESS EMPLOYING A BLEACHING OPERATION (FIG. I)

Absorption [Absorber] Conditions Item Preferred General Lower Section Absorbent 79 to 80% HNO, 75 to HNO, Pressure. psig z 10 I00 1 20 Temp. C. 3 to 10 0 to 15 F. 37 to 50 32 to 59 Vapor Pressure of 0.8 i 0.2 0.8 1- 0.2 Solvent HNO,, psig Upper Section Absorbent 0.1% l-INO, 0.05 to 5% HNO, Pressure. psig 95 I 10 95 x 20 Temp.. C. 20 I 15 30 z 30 F. 41 to 95 32 to 140 Conditions Preferred General Pressure, psig 400 to 600 300 to 1600 Temp.. C. 60 t 10 70 t 30 F. 122 to I58 104 to 212 Conversion of NO, HNO,. 60 t 5 60 2 20 vap rization Bleaching [Bleacher] Conditions Item Preferred General Means air air stripping or similar gas stripping like N,. etc. Pressure. psig i 10 i 25 Temp..C. 60:10 60130 F. 122 to I58 86 to I94 Distillation [Strong HNO, Distillation Column] Conditions Preferred General Feed in Z N03 85 i 5 75 to 95 Overhead. k HNO 95 minimun 90 to I00 Bottoms, 7% HNO;, 79 to 80 75 to 85 Pressure. psig 0 0 z 5 Temp. Bottom 1 I7 3C. 117 3C.

237F. to 248"F. 237F. to 248F. Top 88 to 100C. 88 to 100C.

F. to 212F. 190F. to 212f.

PROCESS EMPLOYING FLASHING OPERATION Reaction [Reactor] vaporization Same as set forth above for the operation employing the bleachers.

[Condensate Flasher Preceding Weak HNO Distillation] Conditions Item Preferred General Feed in 92 l-INO 35 to 45 less than 55 Temp. "C. 100 1 S 100 t 20 PROCESS EMPLOYING FLASHING OPERATION Continued (FIG. ill

The nitrogen-peroxide fed to the Pressure Absorber through line 4 may be from any suitable source, although it preferably is provided by ammonia oxidation as discussed above. All that is required is that the nitrogen peroxide constitute at least about 5% of the feed gas, by volume, the remainder being substantially inert or the nitrogen peroxide precursors nitric oxide and oxygen. Minor amounts of entrained water may be present, but the nitrogen peroxide should be substantially anhydrous, e.g., contain less than about 1% water. When the nitrogen peroxide is provided by ammonia oxidation, it generally will be at a concentration no greater than about l0% and will have a state of oxidation of at least about 90%, i.e., at least about 90% of total nitrogen peroxide plus nitric oxide will be nitrogen peroxide.

The Absorbers may be any desired type of countercurrent scrubbing columns such as, for instance, those packed with chemical-ware rings, or they may be. and preferably are, towers containing a plurality of plates or trays fitted with cooling coils and bubble caps. The Reactor, too, may be of such construction; but, of course, must be of heavy-walled construction to withstand reaction pressures. Also, sieve plates can be employed instead of packing or bubble cap trays. Although countercurrent flow is not required in the Reactor, flow preferably will be as shown on the drawings, and through a column of the bubble cap type. The Bleacher and Strong and Weak HNO; Distillation Columns are of conventional design. Whereas Reboilers and Condensers are shown on the drawing for each distillation column, they may properly be considered part of the distillation columns themselves.

The source of water fed to the Reactor (line 14) is not critical to the process of the instant invention in its broader aspects. For example, part or all of such water could be the product stream from a conventional pressure process nitric acid plant, which would typically contain about 62 to 65% HNO In this case, part of the condensate from the Cooler-Condenser (if any) could be fed to the absorption column of the referenced plant, the remainder being withdrawn from the system for use or distilled as shown on the drawing to give about a 55 to 68% l-lNO product. Alternatively, all of the condensate could be withdrawn or distilled. Cooler- Condenser condensate can be the sole source of water for the Reactor. but some will have to be disposed of in one of the ways just mentioned when the condensate contains more water than required for Reactor operation. In addition, pure water may be fed to the Reactor (line 14), in which case, any condensate will be withdrawn, fed to another plant or distilled. When a Weak HNO Distillation Column is not employed, and the Pressure Absorber contains the preferred scrubbing (upper) sections, it may be serviced with pure water instead of the usual distillate from the Weak HNO; Distillation Column. The resulting aqueous nitric acid effluent from such Absorber section may then be fed to the Reactor to serve as the source for part or all of the reaction water.

in a still further modification of the invention, when the nitrogen peroxide is supplied by an ammonia oxidation process and Cooler-Condenser and Separator designs give a condensate (stream 5) whose concentration is less than about 30% HNO all of the bottoms from the Weak HNO; Distillation Column can be fed to Reactor to supply reaction water. Thus, there is only one product withdrawn from the system, i.e., the strong product containing about HNO or better, and there is no surplus condensate that must be disposed of. This embodiment is particularly useful where there is no on-site need for the weaker product, i.e., acid of less than azeotropic concentration.

in yet another modification the cooler-condensate is bleached, as in the case of the Reactor effluent acid, and the nitrogen peroxide thus recovered fed to the absorber along with Bleacher and Reactor offgas, while the bleached condensate is fed to the Weak HNO Distillation Column for subsequent treatment as shown on the drawing.

The following Examples illustrate the processes of this invention. Examples 1 and 2 are experiments performed to obtain information required for Absorber and Reactor design, respectively. Example 3 illustrates operation of a plant embodying the process illustrated in FIG. I and whose design is based on, inter alia, data shown in Tables I to V]. Example 4 illustrates a preferred plant process according to FIG. II and also is based upon the aforementioned data.

EXAMPLE l ABSORBER DESIGN Aqueous solutions of HNO containing 80% HNO are prepared. N0 is dissolved in this 80% HNO at a temperature of about 5C. and the solutions are placed in stainless steel cylinders. The cylinders are pressurized with nitrogen to psig i 7 psig and maintained at 5C. in a constant temperature bath until the gas and liquid phases reach equilibrium. This usually takes 4-5 hours. During this time, the cylinder is shaken at 30 minute intervals for a period of 1 minute. When equilibrium is obtained, the gas and liquid phases are separated and analyzed. The data in Table I show the results of the Experiments.

TABLE 1 NO, Solubility in 80-8271 HNO (NO, Free Basis), l8-20% H O Solutions at T 5C.

EXAMPLE 2 REACTOR DESIGN TABLE lVContinued Aqueous solutions of l-INO containing about 80% Cmvmio" of HNO; are prepared. N0 is dissolved in the l-INO solu- Run Reactor Temp" (Li) tions, which are then pumped under pressure into the 5 lg 32 42.5 62.6

top of a reactor and flowed countercurrent to a stream of air. The reactor is a 3-inch-diameter vessel with five sieve plates spaced at 13-inch intervals. Liquid holdup TABLE V volume on each plate is approximately 200 ml. The effects 0f the following variables are determined over the Effect of HNO Concentration in Feed Solution on ranges i d: NO Conversion to HNO,

Presure 1475 psig I Liquid Retention Time 90 to I00 min NQzFORCCMmUQB Feed from 22 to 30% y welght Air Rate to Reactor I cc/min measured at 1475 psig Llquld Feed No 29 z m by wt Reactor pressure from 500 to 1500 psrg l5 Temperature 75 Reactor temperature from 25 to 75C. HNO concn. in feed liquid from 78 to 85% by weight (NO; free basis) h H O concn. in feed liquid from l5 to 22% by weig t Li quid retention time in reactor from 26 to 88 minutes ggzgga'sgggifig m ig g z' 2122 0 Run No. Free Basis) The fraction of NO, fed to the reactor which is 2 converted to HNO; is determined by analyzing for 2; gig 3-; HNO;, in the feed and effluent liquid streams. The resuits are shown below in Tables [I to VI.

TABLE V] TABLE II Effect of Liquid Retention Time on NO, Conversion to HNO,1 Effect of NO: Concentration in Feed Liquid on NO, F d "No 79 s 80% (N0 f b C t HNO. ee ree asls o 30 Feed ,0 4 2040591 no, Free basis) X' b Pressure I475 psig 2: 2 (figg rse lias isi Air Rate to Reactor 120 cc/min measured at [475 psig Pressure i480 psig Feed N0, 24 to 29% by wt Liquid Retention Time 80 to 100 minutes Temperature 75C Air Rate to Reactor I20 cc/min measured at 1480 psig Temperature 75C.

. Run Conversion of NO NO Concentration in Conversion of NO to 1 Run N04 1 Feed Liquid "N03 2 No. Liquid Retention Time (min) to HNO,, 5%

i8 26 62.6 (1 by wt) 16 22.6 62.7 40 88 663 50 30.4 62.5 40

TABLE III EXAMPLE 3 PLANT DESIGN The following table gives steady state operating con- Pmsu" N01 Conversm ditions for a nitric acid lant desi nod to roduce 394 P 8 P Feed aNo,= 80% (N0 free basis) tons/day 9 8% l-INO and 56 tons/day of 657; HN0 Feed H,0 20% (N0, fret:J basis) The plant is set up as shown in FIG. 1 and with corre- L' uid Retention Time 8 to I00 minutes Al? rate to Reactor I20 cc/min measured at reactor sp( mdl ngly n'umbered f F A standtld ammoma pressure oxidation umt employing platinum-containing catalyst is: f g f fg P %l is the source of nitrogen peroxide. The Absorber is p Conversion of N0, 12.5 feet in inside diameter, 75 feet high, and contains Ream" Pmsu" (P3181 a total of 60 refrigerant-cooled bubble cap trays spaced 46 480 409 1 foot apart. Fifty of the trays are in the bottom section 49 1000 56.8 and the remaining ten are In the upper section. All trays 50 1500 are of the type conventionally employed in pressure process nitric acid plants.

The Reactor is 10 feet in inside diameter, 50 feet TABLE Iv high, and contains 40 bubble cap trays of the above type spaced 1 foot apart. The top ten trays are cooled Effect Tcmpemu" to 35C. The bottom trays are warmed to 60C. and de- Feed N0a (Nogfree basis) signed to provide a total holdup time of 60 minutes. Feed H,0 20% (N0, free basis) Line 15 enters the Reactor between the tenth and elev- Pressure 1480 ps' Liquid Reemion Time 30 1 5 minutes enth trays (counting from the top) and line 14 enters Air Rate to Reactor H5 to I50 cc/min measured at 5 above the top tray. The Reactor 1s heavy-walled to I430 P518 withstand its operating pressure of 1,500 psig.

Feed N0, 22 to 24% by wt containing catalyst is the source of nitrogen peroxide. The Absorber is 12.5 ft. in inside diameter, 75 ft. high and contains a total of 60 refrigerant-cooled bubble cap trays.

The Reactor is ft. in inside diameter, 65 ft. high and contains 55 bubble cap trays spaced 1 foot apart.

Line 1 2 3 5 6 7 8 Description NH Air Process Process Cooler- Bleacher Reactor Total Feed Fee Gas from Gas C ondenscr Offgas Reactor Recycle Catalytic to Condensate Ol'fgas Gas Converter Absorber lh/hr 10.819 165.169 175.988 149.840 26.149 39.512 32.776 72.288 Temp. C 30 30 900 40 40 60 35 30 Pressure. psig 170 0 116 109 109 110 1 I0 109 Composition. lb/hr NH, 10.819 e O 38.309 13.360 2.709 7.946 2.530 10.476 N 126.861 127.306 127.306 26.313 20.949 47.262 N0 18.138 659 N0,. N 0. 19.166 5.253 9.297 14.550 H,O gas 17.184 H,O (liq) 15.689 "N03 (liq) 10.460

Line l0 ll 12 13 14 15 16 17 Description Absorber Total Weak Acid Weak HNO Weak HNO Reactor Reactor Bleached Tailgas Distillation Recovery Sidestream Feed to Feed Product Reactor Col Feed Stream Product Reactor Solution Product Stream lb/hr 188.121 31.769 2.878 7.190 1 .986 1 4.096 131.580 126.328 Temp. C 50 40 35 11) 60 60 Pressure. psig 102 0.5 0 0 1500 1500 1500 1 10 Composition. lb/hr i 0 13.185 N, 174.569 N0 N0 N 0. 367 36.375 5.252 11,0 gas 11,0 (liq) 18.538 2.849 2.516 4.545 19.890 20.164 20.164 HNO (liq) 13.231 29 4.674 8.441 67.831 106.164 106.164

Line 18 19 20 22 23 Description Recycle Bleacher Reactor Waste Strong HN Ir Air HNQ, HNO, Solvent Feed Product lb/hr 92.832 34.259 7.275 8.714 33.495 Temp. "C 35 65 60 40 Pressure. psig 105 1 12 1500 0 0 Composition. lb/hr z! z 7.946 6.326 26.313 0.949 NO N0 N 0. H 0 gas H O (liq) 19.494 8.6 7 670 N03 (liq) 73.338 87 32.825

The trays are warmed to 60C. and designed to produce a total holdup time in the range 60-120 minutes. Line 29 enters the top of the Reactor above the top tray. The Reactor is heavy walled to withstand its operating pressure of 500 psig. Both the Flashers and the Weak and Strong l-lNO Distillation Columns are of conventional design for operation at atmospheric pressure.

Line 1 2 3 4 5 26 27 7 Description NH,-, Feed Air Feed Process Process C ooler- Vapor From Weak HNQ. Reactor Gas from Gas Condenser Partial Con- Distillation Oil Catalytic to Condensate denser and Column Gas Converter Absorber Separator Feed lh/hr 10.883 155.714 166.598 137.507 29.091 1.374 28.285 27.839 Temp. C. 30 31) 350 60 100 60 Pressure. psig 170 0 125 120 0 l) 121 Composition. lb/hr NH 10.883 0 35.633 10.538 3.328 2.340

Continued Line 1 2 3 4 5 26 27 7 Description NH, Feed Air Feed Process Process Cooler- Vapor From Weak HNO Reac- It)! Gas from Gas Condenser Partial Cun- Distillation Off Catalytic tu Condensate denser and Column Gas Comcrtcr Absorber Separator Feed 151 117.331 117.7140 117.7140 19.264 N0 114.245 7.473 2 N0 N. .o, 7.639 457 457 6.235 H. .O gals 2.750 20.035 1.287 915 H O [ii 1 17.100 16.744 141901117 1 11.534 11.541

Line 10 13 14 214 29 16 Weak Weak Atmo- Pressurc HNO HNO" sphcric Reactor Absorber Sidcstream Feed to Absorber Fced Reactor Description Off Gas Product Reactor Bottoms Solution Product 1671" 139.420 10.906 6.041 29.401 147.572 145.067 Temp. T. 5 35 35 5 5 60 Prcssu re. psig I I0 0 120 I20 500 500 Composition. lh/hr 5 0 1.1459 N, 137.043 NO N0 N. .O, 353 7.646 33.638 7.220 H2O gas 9 14,0 [liq] 3.490 1.933 4.466 25.117 21.389 HMO-.116 7.416 4.1024 17.239 88.8] 7 116.458 HNO.. (gas-l 156 Line I8 36 20 22 37 32 40 34 Recycle Recycle HNO Ott- HNOR Solvent Gas Feed to Vapor Vapor from Solvent to to the from Reactor Strong from Strong Strong HNC), Strong Atmo- Prcssure Atmospheric spheric Air Waste HNOH HNO Partial HNO" Description Absorber Absorber Absorber Feed HNO, Flasher Flasher Condenser Product 113 111 86.205 17,741 93 25.334 10.768 145.068 4.958 5.418 30.744 Temp. C. 5 5 5 80 100 60 74 90 40 Pressure. psig I20 0 0 500 0 (l 0 U U Composition. lblhr o. 16 5.850 14 N: 19,263 NO o 0 0 77 7.220 3.016 4.240 46 H10 gas 22! 2 H,() mm 17.241 3.548 10.763 21.389 614 H190. (li i 68.964 14.193 5 116.458 1.942 30,084 HNCD; (gas) 1.162

What is claimed is: desorbed from the true solution during the reaction 1. A process for producing strong nitric acid having and recycling the gaseous peroxide to be absorbed a concentration of at least about 90% HNO which 50 in step (a); comprise continuously: c. vaporizing a portion of the aqueous nitric acid a. countercurrently absorbing gaseous nitrogen perstream from step (b) to remove at least 50% nitrooxide in aqueous nitric acid bottoms from step (e) gen peroxide contained therein; and at a temperature and pressure of about from 0 to d. absorbing the nitrogen peroxide obtained in step l5C. and 80 to I20 psig. respectively, thereby (c) in a portion of the bottoms from step (e); and forming a true solution of aqueous nitric acid have. distilling the aqueous nitric acid from step (c) at ing a concentration of about from 75 to 85% HNO about atmospheric pressure and recovering as dison a nitrogen peroxide free basis and about from tillate strong nitric acid having a concentration of 15 to 30% dissolved nitrogen peroxide, based on about from 90 to 100% HNO leaving as bottoms the total weight of the solution; aqueous nitric acid having a concentration of about b. reacting the true solution of step (a) with molecular oxygen and water at a temperature in the range of about 40 to I00C. and a pressure in the range of about 300 to 1,600 psig, thereby increasing the concentration of the aqueous nitric acid of step (a) to a higher concentration of about 80 to 90% HNO in the form of an aqueous HNO bottoms stream, while recovering gaseous nitrogen peroxide from to HNO 2. A process of claim 1 wherein the water required in step (b) is supplied by aqueous nitric acid having a concentration below about 68.5% HNO:,.

3. A process of claim 2 wherein the aqueous nitric acid supplied to step (b) first absorbs some of the nitrogen peroxide desorbed in step (b), the remaining desorbed N0 being recycled to step (a), and thereafter joins said true solution for combined flow countercurrently to a stream of gas containing molecular oxygen.

4. A process of claim I wherein step (b) comprises reacting the true solution with the molecular oxygen in air flowing countercurrently to said solution.

5. A process of claim 1 wherein step (c) is accomplished by flashing about I-I% by weight of the aqueous nitric acid reaction product of step (b).

6. A process of claim 1 which additionally comprises continuously:

g. scrubbing the gaseous effluents from absorption step (a) with water to recover unabsorbed gaseous nitrogen peroxide and HNO vapor as weak nitric acid; and

h. distilling the weak nitric acid from step (g) to give as distillate aqueous nitric acid having a concentration below about HNO and as bottoms aqueous nitric acid having a concentration of about from 55 to 68% HNO;,.

7. A process of claim 1 wherein the initial source of nitrogen peroxide for step (a) is a substantially anhydrous gaseous mixture which is prepared by catalytically oxidizing ammonia with the molecular oxygen in air, cooling the resulting gases to condense water of reaction and oxidize nitric oxide to nitrogen peroxide, and separating water of reaction as weak aqueous nitric acid condensate having a concentration less than bout 55% HNO and which additionally comprises continuously:

h. distilling such aqueous nitric acid condensate having a concentration of less than about 55% HNO to give as distillate aqueous nitric acid having a concentration below about 5% HNO and as bottoms aqueous nitric acid having a concentration of about from 55 to 68% HNO;,.

8. The process of claim 7 which additionally comprises continuously i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as a source of said Water.

9. a process of claim 8 which additionally comprises j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water.

10. A process of claim 1 in which the absorption of step (d) is preformed at substantially atmospheric pressure.

II. A process of claim 10 in which the vaporizing of step (c) is performed by depressurizing the aqueous HNO bottoms stream of step (b) to substantially atmospheric pressure.

12. A process of claim 11 which comprises additionally heating the depressurized stream, and again depressurizing the stream to remove substantially all the remaining dissolved nitrogen peroxide.

13. A process of claim 1 wherein the initial source of nitrogen peroxide for step (a) is a substantially anhydrous gaseous mixture which is prepared by catalytically oxidizing ammonia with the molecular oxygen in air, cooling the resulting gases to condense water of reaction and oxidize nitric oxide to nitrogen peroxide, and separating water of reaction as weak aqueous nitric acid condensate having a concentration less than about 55% HNO wherein the absorption of step (d) is performed at substantially atmospheric pressure, and which additionally comprises continuously:

g. scrubbing the gaseous effluents from absorption step (a) with water to recover unabsorbed gaseous nitrogen peroxide and HNO vapor as weak nitric acid;

h. distilling the weak nitric acid from step (g) and the aqueous nitric acid condensate having a concentration of less than about 55% HNO to give as distillate aqueous nitric acid having a concentration below about 5% HNO; and as bottoms aqueous nitric acid having a concentration of about from 55 to 63% HNO i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as a source of said water; and

j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water.

14. A process of claim 1 wherein the vaporizing of step (c) is done by depressurizing the aqueous nitric acid from step (h) to about I05 to I55 psig and bleaching such depressurized acid at a temperature and pressure of about from 30 to 90C. and I05 to [55 psig, respectively, to recover substantially the remaining nitrogen peroxide dissolved in the aqueous nitric acid of step (b).

15. A process of producing strong nitric acid having a concentration of at least about 90% HNO which comprises continuously:

a. countercurrently absorbing gaseous nitrogen peroxide in agueous nitric acid bottoms from step (f) at a temperature and pressure of about from 0 to 15C. and to I20 psig, respectively. thereby forming a true solution of aqueous nitric acid having a concentration of about from 75 to HNO and about from I5 to 30% dissolved nitrogen peroxide, based on the total weight of the solution, wherein the initial source of nitrogen peroxide is a substantially anhydrous gaseous mixture which is prepared by catalytically oxidizing ammonia with the molecular oxygen in air, cooling the resulting gases to condense water of reaction and oxidize nitric oxide to nitrogen peroxide, and separating water of reaction as weak aqueous nitric acid condensate having a concentration less than about 55% HNO b. reacting the true solution of step (a) with molecular oxygen and water at a temperature and pressure of about from 40 to I00C. and 300 to 1,600 psig, respectively, thereby increasing the concentration of the aqueous nitric acid of step (a) to a higher concentration of about from 80 to HNO while recovering gaseous nitrogen peroxide desorbed from the true solution during the reaction;

c. depressurizing the aqueous nitric acid from step (b) to about from I05 to 155 psig;

d. bleaching the depressurized acid of step (c) at a temperature and pressure of about from 30 to 90C. and to psig, respectively, to recover substantially the remaining nitrogen peroxide dissolved in the aqueous nitric acid of step (b); recycling the nitrogen peroxide recovered in steps (b) and (d) to step (a) for admixture with fresh nitrogen peroxide and absorption in said bottoms; and f. distilling the bleached aqueous HNO from step (d) at about atmospheric pressure and recovering as distillate strong nitric acid having a concentration of about from 90 to 100% HNO leaving as bottoms aqueous nitric acid having a concentration of about from 75 to 85% HNO respectively;

i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as the source of said water; and

j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water. 

1. A PROCESS FOR PRODUCING STRONG NITRIC ACID HAVING A CONCENTRATION OF AT LEAST ABOUT 90% HNO3 WHICH COMPRISE CONTINUOUSLY: A. COUNTERCURRENTLY ABSORBING GASEOUS NITROGEN PEROXIDE IN AQUEOUS NITRIC ACID BOTTOMS FROM STEP (E) AT A TEMPERATURE AND PRESSURE OF ABOUT FROM 0* TO 15*C. AND 80 TO 120 PSG, RESPECTIVELY, THEREBY FORMING A TRUE SOLUTION OF AQUEOUS NITRIC ACID HAVING A CONCENTRATION OF ABOUT FROM 75 TO 85% HNO3 ON A NITROGEN PEROXIDE FREE BASIS AND ABOUT FROM 15 TO 30% DISSOLVED NITROGEN PEROSIDE, BASED ON THE TOTAL WEIGHT OF THE SOLUTION; B. REACTING THE TRUE SOLUTION STEP(A) WITH MOLECULAR OXYGEN AND WATER AT A TEMPERATURE IN THE RANGE OF ABOUT 40* TO 100*C. AND A PRESSURE IN THE RANGE OF ABOUT 300 TO 1,600 PSIG, THEREBY INCREASING THE CONCENTRATION OF THE AQUEOUS NITRIC ACID OF STEP (A) TO A HIGHER CONCENTRATION OF ABOUT 80 TO 90% HNO3 IN THE FORM OF AN AQUEOUS HNO3 BOTTOMS STREAM, WHILE RECOVERING GASEOUS NITROGEN PEROXIDE DESORBED FROM THE TRUE SOLUTION DURING THE REACTION AND RECYCLING THE GASEOUS PEROXIDE TO BE ABSORBED IN STEP (A); C. VAPORIZING A PORTION OF THE AQUEOUS NITRIC ACID STREAM FROM STEP (B) TO REMOVE AT LEAST 50% NITROGEN PEROXIDE CONTAINED THEREIN; AND D. ABSORBING THE NITROGEN PEROXIDE OBTAINED IN STEP (C) IN A PORTION OF THE BOTTOMS FROM STEP (E); AND E. DISTILLING THE AQUEOUS NITRIC ACID FROM STEP (C) AT ABOUT ATMOSPHERIC PRESSURE AND RECOVERING AS DISTILLATE STRONG NITRIC ACID HAVING A CONCENTRATION OF ABOUT FROM 90 TO 100% HNO3, LEAVING AS BOTTOM AQUEOUS NITRIC ACID HAVING A CONCENTRATION OF ABOUT FROM 75 TO 85% HNO3.
 2. A process of claim 1 wherein the water required in step (b) is supplied by aqueous nitric acid having a concentration below about 68.5% HNO.sub.3.
 3. A process of claim 2 wherein the aqueous nitric acid supplied to step (b) first absorbs some of the nitrogen peroxide desorbed in step (b), the remaining desorbed NO.sub.2 being recycled to step (a), and thereafter joins said true solution for combined flow countercurrently to a stream of gas containing molecular oxygen.
 4. A process of claim 1 wherein step (b) comprises reacting the true solution with the molecular oxygen in air flowing countercurrently to said solution.
 5. A process of claim 1 wherein step (c) is accomplished by flashing about 1- 10% by weight of the aqueous nitric acid reaction product of step (b).
 6. A process of claim 1 which additionally comprises continuously: g. scrubbing the gaseous effluents from absorption step (a) with water to recover unabsorbed gaseous nitrogen peroxide and HNO.sub.3 vapor as weak nitric acid; and h. distilling the weak nitric acid from step (g) to give as distillate aqueous nitric acid having a concentration below about 5% HNO.sub.3 and as bottoms aqueous nitric acid having a concentration of about from 55 to 68% HNO.sub.3.
 7. A process of claim 1 wherein the initial source of nitrogen peroxide for step (a) is a substantially anhydrous gaseous mixture which is prepared by catalytically oxidizing ammonia with the molecular oxygen in air, cooling the resulting gases to condense water of reaction and oxidize nitric oxide to nitrogen peroxide, and separating water of reaction as weak aqueous nitric acid condensate having a concentration less than bout 55% HNO.sub.3, and which additionally comprises continuously: h. distilling such aqueous nitric acid condensate having a concentration of less than about 55% HNO.sub.3 to give as distillate aqueous nitric acid having a concentration below about 5% HNO.sub.3 and as bottoms aqueous nitric acid having a concentration of about from 55 to 68% HNO.sub.3.
 8. The process of claim 7 which additionally comprises continuously i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as a source of said water.
 9. a process of claim 8 which additionally comprises j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water.
 10. A process of claim 1 in which the absorption of step (d) is preformed at substantially atmospheric pressure.
 11. A process of claim 10 in which the vaporizing of step (c) is performed by depressurizing the aqueous HNO.sub.3 bottoms stream of step (b) to substantially atmospheric pressure.
 12. A process of claim 11 which comprises additionally heating the depressurized stream, and again depressurizing the stream to remove substantially all the remaining dissolved nitrogen peroxide.
 13. A process of claim 1 wherein the initial source of nitrogen peroxide for step (a) is a substantially anhydrous gaseous mixture which is prepared by catalytically oxidizing ammonia with the molecular oxygen in air, cooling the resulting gases to condense water of reaction and oxidize nitric oxide to nitrogen peroxide, and separating water of reaction as weak aqueous nitric acid condensate having a concentration less than about 55% HNO.sub.3, wherein the absorption of step (d) is performed at substantially atmospheric pressure, and which additionally comprises continuously: g. scrubbing the gaseous effluents from absorption step (a) with water to recover unabsorbed gaseous nitrogen peroxide and HNO.sub.3 vapor as weak nitric acid; h. distilling the weak nitric acid from step (g) and the aqueous nitric acid condensate having a concentration of less than about 55% HNO.sub.3 to give as distillate aqueous nitric acid having a concentration below about 5% HNO.sub.3 and as bottoms aqueous nitric acid having a concentration of about from 55 to 68% HNO.sub.3. i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as a source of said water; and j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water.
 14. A process of claim 1 wherein the vaporizing of step (c) is done by depressurizing the aqueous nitric acid from step (h) to about 105 to 155 psig and bleaching such depressurized acid at a temperature and pressure of about from 30.degree. to 90.degree.C. and 105 to 155 psig, respectively, to recover substantially the remaining nitrogen peroxide dissolved in the aqueous nitric acid of step (b).
 15. A process of producing strong nitric acid having a concentration of at least about 90% HNO.sub.3 which comprises continuously: a. countercurrently absorbing gaseous nitrogen peroxide in agueous nitric acid bottoms from step (f) at a temperature and pressure of about from 0.degree. to 15.degree.C. and 80 to 120 psig, respectively, thereby forming a true solution of aqueous nitric acid having a concentration of about from 75 to 85% HNO.sub.3 and about from 15 to 30% dissolved nitrogen peroxide, based on the total weight of the solution, wherein the initial source of nitrogen peroxide is a substantially anhydrous gaseous mixture which is prepared by catalytically oxidizing ammonia with the molecular oxygen in air, cooling the resulting gases to condense water of reaction and oxidize nitric oxide to nitrogen peroxide, and separating water of reaction as weak aqueous nitric acid condensate having a concentration less than about 55% HNO.sub.3 ; b. reacting the true solution of step (a) with molecular oxygen and water at a temperature and pressure of about from 40.degree. to 100.degree.C. and 300 to 1,600 psig, respectively, thereby increasing the concentration of the aqueous nitric acid of step (a) to a higher concentration of about from 80 to 90% HNO.sub.3, while recovering gaseous nitrogen peroxide desorbed from the true solution during the reaction; c. depressurizing the aqueous nitric acid from step (b) to about from 105 to 155 psig; d. bleaching the depressurized acid of step (c) at a temperature and pressure of about from 30.degree. to 90.degree.C. and 105 to 155 psig, respectively, to recover substantially the remaining nitrogen peroxide dissolved in the aqueous nitric acid of step (b); e. recycling the nitrogen peroxide recovered in steps (b) and (d) to step (a) for admixture with fresh nitrogen peroxide and absorption in said bottoms; and f. distilling the bleached aqueous HNO.sub.3 from step (d) at about atmospheric pressure and recovering as distillate strong nitric acid having a concentration of about from 90 to 100% HNO.sub.3, leaving as bottoms aqueous nitric acid having a concentration of about from 75 to 85% HNO.sub.3, respectively; g. scrubbing the gaseous effluent from absorption step (a) with water to recover unabsorbed gaseous nitrogen peroxide and HNO.sub.3 vapor as weak nitric acid: h. distilling the weak acid from step (g) and the aqueous nitric acid condensate having a concentration below about 5% HNO.sub.3 and as bottoms aqueous nitric acid having a concentration of about from 55 to 68%. i. feeding at least a portion of the bottoms from step (h) to step (b) to serve as the source of said water; and j. feeding at least a portion of the distillate from step (h) to step (g) to serve as a source of said water. 